1. Field of the Invention
The present invention relates to a toner for use in an image-forming method such as an electrophotographic method, an electrostatic recording method, an electrostatic printing method, or a toner jet type recording method.
2. Description of the Related Art
Image-forming apparatuses have already entered widespread use as information output instruments linked to other information instruments because digitalization of the apparatuses is now able to store information as a digital signal. In addition, each of the image-forming apparatuses has been strongly requested to, for example, be available at a reduced cost and correspond to various media owing to the diversification of methods by which a user uses the apparatus as well as to show, for example, the following ability: the apparatus can form a high-definition, high-grade, high-quality image at a high speed with high reliability.
The achievement of a reduction in cost at which an image-forming apparatus is available ineluctably requires the simplification of each image-forming process in the apparatus; particularly in the case of a high-priced machine having high productivity, various functions that can be mounted on the machine must be omitted to the extent possible for reducing the cost for the machine. On the other hand, in order that an image-forming apparatus may be able to cater to the demands for a high grade, high image quality, high reliability, and the like, the number of characteristics requested of toner and each key part in the apparatus has been increasing without cessation.
In addition, problems that previously hardly came to the surface heretofore have started to become evident owing to the diversification of: areas where users use image-forming apparatuses; environments under which the users use the apparatuses; and methods by which the users use the apparatuses.
For example, some problems occur remarkably when paper having a high degree of surface unevenness such as recycled paper or bond paper is used, though the problems are hardly regarded as being troublesome when relatively smooth paper such as coated paper for color printing is used.
One of the problems is electrostatic offset. The electrostatic offset hardly occurs on paper having high surface smoothness such as coated paper, but is apt to occur on paper having a high degree of surface unevenness such as recycled paper.
The electrostatic offset means the following phenomenon: upon passage of paper onto which an unfixed toner image has been transferred through a fixing nip portion between the fixing film and pressure roller of a fixing unit, partially molten toner present at a depressed portion of the paper flies toward the fixing film, and, after the fixing film has rotated once, the toner that has flown toward the fixing film fixes on the paper. The phenomenon becomes remarkable under a low-temperature, low-humidity environment; the phenomenon becomes additionally remarkable particularly in an isolated dot image disadvantageous in terms of fixing performance on paper because an adhesive force between toner hardly acts in the image.
Similarly, fixation tailing is apt to occur on paper having a high degree of surface unevenness. The fixation tailing is the following phenomenon: upon entry of paper onto which unfixed toner line images have been transferred into a fixing nip portion, water vapor in the paper explodes at a depressed portion of the paper to blow off a subsequent line image, with the result that the image tails. The phenomenon becomes remarkable particularly under a high-temperature, high-humidity environment; the phenomenon becomes additionally remarkable on paper that has been left to stand under a high-temperature, high-humidity environment. In addition, those problems are apt to be remarkable in a machine having a high process speed where the fixing performance of toner is apt to deteriorate because the problems are apt to be additionally remarkable when a toner image on paper is not completely molten.
Such problems relating to the fixation of toner can be reduced by changing the constitution of a fixing unit or controlling a fixing bias. However, measures against the constitution of the main body of an image-forming apparatus are not preferable because the measures are in a trade-off relationship with such reduction in cost for the apparatus as described above.
Meanwhile, investigations on a charge control resin as a charge control agent have been underway in recent years from the viewpoints of the control of triboelectric charging and safety. For example, a method involving the use of a polymer of a styrene-based monomer and 2-acrylamide-2-methylsulfonic acid has been disclosed (Patent Document 1). In addition, a method involving the use of a polymer of a styrene-based monomer and 2-acrylamide-2-methylsulfonic acid as a charge control agent for a polyester resin has been disclosed (Patent Document 2). Further, a toner containing a sulfonic group-containing (meth) acrylamide copolymer having a specific glass transition temperature as a charge control agent has been disclosed (Patent Document 3). However, none of those documents refers to electrostatic offset and tailing in a fixing process, though those methods and toner have the following characteristic in common: excellent triboelectric charge-providing performance.
In addition, attempts have been made to use two kinds, i.e., any such charge control resin as described above and another charge control agent in combination with a view to achieving an additional improvement in developing performance of toner. For example, attempts have been made to improve the charging performance and developing performance of toner by using a charge control resin and an aromatic oxycarboxylic acid-based charge control agent in combination (Patent Documents 4 and 5). In addition, attempts have been made to improve the developing performance of toner under a high-temperature, high-humidity environment by controlling a monomer amount in a charge control resin (Patent Document 6). Further, attempts have been made to achieve an additional improvement in performance of a toner containing a charge control resin by paying attention to the dielectric characteristic of the toner (Patent Documents 7 and 8). Meanwhile, a certain document describes an improving effect of, for example, a system obtained by further incorporating an azo-based iron compound into a toner containing a charge control resin on the developing performance of the toner by paying attention to the moisture absorption of the toner (Patent Document 9).
All of those attempts are aimed at an effect in a developing portion in an image-forming process, and an improving effect on the developing performance of toner has been actually observed in each of the attempts. However, none of the documents refers to the behavior of the toner before and after its passage through a fixing nip, so the documents are susceptible to improvement in terms of electrostatic offset and tailing each having the following characteristic: the behavior of the toner in a fixing process rather than in a developing process plays a pivotal role in its occurrence.
The following fact has been generally known: in the case of a magnetic toner for use in a one-component developing system advantageous for a reduction in size of an image-forming apparatus, the states of dispersion of magnetic iron oxide particles in the magnetic toner and the physical properties of the iron oxide particles themselves significantly affect, for example, various characteristics requested of the toner, such as a developing characteristic and durability, and the deterioration of the toner.
For example, when the magnetic iron oxide particles are insufficiently dispersed in the particles of the magnetic toner, the total amounts of the magnetic iron oxide particles exposed to the surfaces of the toner particles vary from toner particle to toner particle. When the amount of the magnetic iron oxide particles on the surface of a toner particle is small, upon triboelectric charging with a charge-providing member (which may hereinafter be referred to as “developing sleeve”), the surface of the toner particle is charged to a large extent, and in some cases, the surface is excessively charged. In contrast, when the magnetic iron oxide particles are present in an excessive amount on the surface of a toner particle, the charge of the toner is apt to leak from the magnetic iron oxide particles. As a result, the toner hardly obtains a large charge quantity. In addition, toner with reverse polarity is apt to generate owing to contact between any such magnetic substance particle and a binder resin, so the charging distribution of the toner is apt to widen.
When the charge of the toner is apt to leak from the above magnetic iron oxide particles and the charge quantity distribution of the toner is wide, the charging stability of the toner is apt to be insufficient in the latter half of the service life of a recent long-life cartridge. Accordingly, particularly when the cartridge is left to stand in the latter half of its service life, a reduction in density and fogging at the time of the start-up of the image-forming apparatus in the morning are apt to be remarkable after the cartridge has been left to stand overnight.
The phenomena are apt to be additionally remarkable when a large number of images each having a relatively low print percentage are printed with a large-capacity cartridge filled with a large amount of toner because the charge quantity distribution of the toner fed to the developing sleeve is apt to widen.
In particular, the electrostatic offset and the fixation tailing described above are apt to be worse when the charge of the toner on paper is apt to leak. Therefore, as described above, it is important to control the states of dispersion of the magnetic iron oxide particles in the toner and the physical properties of the magnetic iron oxide particles themselves.
A large number of proposals have been conventionally made on improvements in flowability and resistance of a magnetic iron oxide in a magnetic toner through the incorporation of a specific element into the surface and inside of the magnetic iron oxide.
For example, a magnetic particle powder having the following characteristics has been disclosed (Patent Document 10): the powder contains 0.10 to 1.00 mass % of Si, a coprecipitate of silica and alumina is present on the surface of the powder, and further, an oxide particle or water-containing oxide particle of an element selected from Fe, Ti, Zr, Si, and Al adheres onto the coprecipitate.
In addition, an iron oxide particle having the following characteristic has been disclosed (Patent Document 11): the particle is coated with a composite iron oxide layer containing Ti and Fe.
Further, a toner containing a magnetic iron oxide having the following characteristics has been disclosed (Patent Document 12): the magnetic iron oxide contains at least Si, Zn, and Ti, and the dissolution ratio of each of the elements when a portion ranging from the surface of the magnetic iron oxide to a depth corresponding to 5% of the radius of the magnetic iron oxide is dissolved is specified.
Each of those attempts has achieved improvements in flowability and resistance of a magnetic substance to some extent through the localization of Ti or Zn on the outermost surface layer of the magnetic substance. However, the attempts are insufficient in terms of the control of the dielectric loss tangent of toner largely involved in electrostatic offset and tailing.
In addition, a magnetite particle having the following characteristics has been disclosed (Patent Document 13): the particle contains a silicon component continuously from its center to its surface, its outer shell is coated with a metal compound formed of at least one kind of a metal component selected from Zn, Mn, Cu, Ni, Co, Cr, Cd, Al, Sn, Mg, and Ti bonded to the silicon component, and the particle is provided with such a gradient that the concentration of the above metal component with respect to Fe in the outer shell portion of the particle is higher than that in the inner shell portion of the particle and such concentration in the surface layer portion of the particle is additionally high. Further, an iron oxide particle having the following characteristic has been disclosed (Patent Document 14): a core particle from which a silicon component is exposed is coated with an Al component.
Each of those attempts exerts its effect by controlling the abundance of an element when a portion ranging from the surface of any such particle to a depth corresponding to 20% or 40% of the radius of the particle is dissolved. However, the attempts are insufficient to alleviate electrostatic offset and fixation tailing particularly on paper having a high degree of surface unevenness because the physical properties of the vicinity of the surface of toner ranging from the surface to a depth corresponding to about 10% of the radius of the toner play an important role in controlling the dielectric loss tangent of the toner largely involved in electrostatic offset and tailing.
That is, at present, a toner containing a magnetic iron oxide and having the following characteristics has been absent: the toner can of course maintain its excellent charging stability irrespective of an environment under which the toner is used, and sufficient measures against electrostatic offset and fixation tailing are taken by paying attention to a fixing process.
Patent Document 1: JP 63-184762 A
Patent Document 2: JP 3-161761 A
Patent Document 3: JP 2000-56518 A
Patent Document 4: JP 2006-113313 A
Patent Document 5: JP 2006-47367 A
Patent Document 6: JP 2003-255575 A
Patent Document 7: JP 2004-157342 A
Patent Document 8: JP 2002-341598 A
Patent Document 9: JP 2004-78055 A
Patent Document 10: JP 7-240306 A
Patent Document 11: JP 2004-161551 A
Patent Document 12: JP 2004-354810 A
Patent Document 13: JP 3224774 B
Patent Document 14: JP 3544316 B